where \(E^\ast _l\)  and \(E^\ast _\pi \)  are the lepton and pion energies in the kaon rest frame;
\(t\)  is the 4-momentum transfer to the leptonic system;
\(N\)  is a numerical factor; \(f_-(t) = (f_0(t) - f_+(t))(m_K^2 -m_{\pi ^0}^2)/t\) ;
\(m_K\)  and \(m_{\pi ^0}\)  are the
charged kaon and neutral pion masses [1]}. The kinematic factors are

A1  =  mK   (2 El E- mK(E,max- E)) + ml2   ((E,max- E)/4 - E),
